There is increasing interest in MOS integrated circuits in which the component devices have gate dimensions as small as 0.25 .mu.m or even less. Devices having such small dimensions suffer from certain problems that are not of serious concern when the gate dimensions are 1 .mu.m or more. For example, the scaling rules that apply to these small devices call for very thin gate oxide layers, typically equivalent to 30 .ANG.-70 .ANG. of silicon dioxide. Conventional gate oxide layers, which consist of thermally grown silicon dioxide, may be inadequate in several respects when they are made this thin.
For example, such thin oxide layers tend to exhibit a high density of pinholes. These layers are also very permeable to boron. As a result, boron from a p.sup.+ -doped polysilicon gate electrode can readily penetrate the thin oxide layer and contaminate the underlying channel during subsequent, high-temperature processing.
Difficulties also arise in the fabrication of these thin silicon dioxide layers. That is, these layers are conventionally grown at temperatures greater than 900.degree. C., and typically 1000.degree. C. or more. The ability to control the thickness of these layers depends on the growth rate. In general, oxidation at these temperatures proceeds so quickly that it is difficult to assure that the resulting layer will have a uniform thickness. The oxidation rate can, of course, be decreased by reducing the temperature to, e.g., 900.degree. C. or less. However, the resulting films tend to have large compressive stress that enhances the accumulation of electric charge at the semiconductor interface, and may also enhance current leakage.
It is known that silicon oxynitride films are less permeable to diffusing boron atoms than silicon dioxide films. Silicon oxynitride films have, in fact, been made for use as gate dielectrics. However, the interfaces between these films and the underlying semiconductor are generally of poor quality, as evidenced by low carrier mobilities in the resulting device channels. Moreover, these films must generally be made at a relatively high reaction temperature, resulting in poor control over film thickness, as discussed above.
To solve at least some of these problems, several investigators have proposed the use of composite dielectric films that include a layer of silicon dioxide as well as a layer of silicon oxynitride. These composite films are made by thermally growing a thin layer of silicon dioxide, and then forming a layer of silicon oxynitride or silicon nitride by chemical vapor deposition (CVD). However, these approaches do not solve the problem of controlling the thickness of the silicon dioxide film. Moreover, they may result in a relatively high fixed charge at the semiconductor-dielectric interface, and they may result in an interface of poor quality.
U.S. Pat. No. 4,621,277, issued to T. Ito et al. on Nov. 4, 1986, describes a silicon nitride or oxynitride gate dielectric made in three stages. First, the silicon surface is subjected to direct thermal nitridation. The resulting nitride film is then at least partially oxidized. Further growth of the film occurs during this oxidation stage. In the last stage, the oxidized film is subjected to direct thermal nitridation. Penetration of the oxidized film by nitrogen atoms during this stage leads to further nitridation of the silicon surface.
Although the Ito process reportedly yields a dielectric of high quality, it results in a nitride or oxynitride region, and not a silicon dioxide region, at the semiconductor interface. Moreover, this method may reduce mobility at the semiconductor-dielectric interface.
Thus, practitioners have hitherto failed to provide a dielectric that combines the advantages of silicon oxynitride with the desirable interfacial properties of silicon dioxide, while preserving high quality and tight control over film thickness.